In a mobile radio, radio signals are subject to multipath propagation, that is to say a number of versions of the received signal occur in the receiver as a result of reflection, scatter and diffraction of the transmitted radio signal on various obstructions in the transmission path and these versions are shifted in time with respect to one another, and have different attenuations. The method of operation of a rake receiver is based on the idea of the received signal versions which have the highest energy being evaluated separately in rake fingers, and then being superimposed with the correct timings. Each rake finger has an associated path delay, and the oversampled, digitized received signal values, which are stored in a RAM input memory, are input into the rake finger with a delay corresponding to the path delay. In addition, the rake finger has an interpolator for varying the sampling clock rate and thus for fine adjustment to the time delay, with an error signal being supplied to the interpolator from an early/late correlator.
Before the rake fingers in the rake receiver circuit can be set up, a delay time estimate is made, in order to obtain a signal power delay profile (pdp) in which the received signal power is plotted continuously against the delay time, and from which the various propagation paths and the associated delay times can be taken. Pilot symbols (common or dedicated pilot symbols) are transmitted at the transmitter end for this delay time estimate, and the received pilot symbols can be evaluated by correlations with the pilot symbols that are known to the receiver. In this case, at least one product correlation sequence comprising a scrambling code, channelization code and the pilot symbols is used in the receiver. The propagation paths which can be drawn from the power delay profile are then also subjected to a suitable selection process and, finally, are passed to the various rake fingers.
By way of example, FIG. 1 shows an apparatus, which operates on this principle and has a rake receiver and devices for determination and selection of the transmission paths. The sampled and digitized received signal values are supplied to a pulse-shaping filter 1, for example, a root cosine filter and are then supplied not only to a rake receiver 5 but also to a delay time estimator 2. A power delay profile pdpest(k) is determined in the delay time estimator 2 by means of correlation procedures and, possibly, further averaging processes. This power delay profile pdpest(k) is supplied to a path detection and selection unit 3, in which the strongest paths are determined, and those paths which can be assigned to the rake fingers are selected from them. These selected paths are transmitted to a finger allocation unit 4 in which the paths are allocated to specific rake fingers, on the basis of their path position, that is to say their delay time and their signal strength, that is to say their path weight. The finger allocation unit 4 transmits appropriate information about the allocation process between the paths and the rake fingers to the rake receiver 5, which has a number N of rake fingers 5.1 . . . 5.N, in which the received signal values produced by the pulse shaping filter 1 have appropriate delay times added to them, and are then demodulated. The various delay times in the rake fingers are indicated in the drawing by a spatial offset (which increases in decreasing sequence) for the boxes associated with the rake fingers. The demodulated output signals from the rake fingers 5.1 . . . 5.N are supplied to an adder 6, in which, for example, maximum ratio combining (MRC) is carried out. The signal components received via the various transmission paths are superimposed again, with the correct timings, in the adder 6, and a soft output data symbol is emitted from the adder 6.
Since the transmission paths between the transmitter and receiver can change very quickly, the delay time estimation and finger allocation have to react sufficiently quickly to avoid any loss of relevant paths. At the same time it is necessary to ensure that only the most or more relevant paths are processed in the rake receiver for each time, since the rake receiver has only a restricted number of rake fingers. On the one hand, it is generally necessary to minimize the probability of the rake fingers having excessively noisy transmission paths applied to them while, on the other hand, it is generally necessary to minimize the probability of useable transmission paths with low noise not being detected.
In the past, it has been known for a power delay profile to be created in a receiver circuit having a rake receiver section, for the local maxima in this delay profile to be determined, and for a number of relevant transmission paths to be selected from these local maxima. However, the previously known methods for delay time estimation and path selection have ignored the instantaneous operating situation, in particular the relative speed between the transmitter and receiver, the frequency offset and the noise level. The correlation and averaging procedures to be carried out in the receiver, as well as the subsequent path selection are, according to the known method, carried out using fixed parameters independently of the respective operating situation. Major parameters are, for example, the correlation length, that is to say the length of successive pilot symbols which are correlated with the received signal in the receiver, the number of correlation results over which the averaging process is carried out, and the number of delay profiles on which further evaluations, such as selections, are carried out. If these parameters are set to fixed values, then this leads to noisy delay profile estimates, or delay profile estimates that are subject to errors, and, as a consequence of this, to incorrect allocations of rake fingers with unusable transmission paths, or to allocations of usable transmission paths which are not feasible.